1. Field of the Invention
This invention relates generally to electronic devices and methods for wireless data transmission and inductive powering of wireless data transmission devices. More particularly, the invention relates to implantable devices and methods for detecting and measuring certain characteristic parameters of a medium, such as, for example, the human body.
2. Background Art
U.S. Pat. No. 5,517,313, the disclosure of which is incorporated herein by reference, describes a fluorescence sensing device comprising a layered array of a fluorescent indicator molecule-containing matrix (hereafter “fluorescent matrix”), a high-pass filter and a photodetector. In this device, a light source, preferably a light-emitting diode (“LED”), is located at least partially within the indicator material, such that incident light from the light source causes the indicator molecules to fluoresce. The high-pass filter allows emitted light to reach the photodetector, while filtering out scattered incident light from the light source. An analyte is allowed to permeate the fluorescent matrix, changing the fluorescent properties of the indicator material in proportion to the amount of analyte present. The fluorescent emission is then detected and measured by the photodetector, thus providing a measure of the amount or concentration of analyte present within the environment of interest.
One advantageous application of a sensor device of the type disclosed in the '313 patent is to implant the device in the body, either subcutaneously or intravenously or otherwise, to allow instantaneous measurements of analytes to be taken at any desired time. For example, it is desirable to measure the concentration of oxygen in the blood of patients under anesthesia, or of glucose in the blood of diabetic patients.
Because of the size and accessibility constraints on a sensor device implanted in the body, there are a number of issues associated with the production of a commercial unit, wherein the need for miniaturization gives rise to reliability, manufacturing cost-effectiveness, and performance concerns. For example, providing the sensing device with data transmission circuitry and/or a power supply would increase the required size of the device with respect to implantation in the body.
A processing system for processing the output signals of a sensor implanted in the body without the need for data transmission circuitry or an internal power supply is taught by U.S. Pat. No. 6,400,974, the disclosure of which also is incorporated herein by reference in its entirety. The '974 patent teaches a processing circuit that powers the sensor through inductively coupled RF energy emitted by the processing circuit. The processing circuit receives data transmissions from the implanted sensor as variations in the load on the processing circuit. The RF energy coupling and data transfer are accomplished by providing two coils: a small coil within the implanted sensor device, and a larger coil connected to the external processing circuit.
One possible implementation of the small coil would be to use a wire-wound coil having a discrete ferrite core attached to a printed circuit board (PCB). While such an implementation works acceptably, improvements can be made.
For example, the attachment of the discrete wire-wound ferrite core to the PCB may be difficult and give rise to reliability issues, resulting in low manufacturing yields.
Second, because each wire-wound coil from the manufacturer is slightly different in characteristics due to manufacturing tolerances, it may be necessary to individually tune each sensor device to properly match the frequency of operation with the associated antenna.
Additionally, the physical structure of the wire-wound coil creates a significant amount of void space within the volumetric displacement of the sensor device, which may result in bubble formation within the required polymeric encasement of the electronic circuit, resulting in a failed device.
A further issue is the requirement for axial alignment of the wire-wound coil ferrite core with the PCB. Alignment of the bent wire tether leads provided on the coil to be attached to the PCB may result in a radial size of the sensor package that is larger than necessary or desired for implantation applications.
Finally, the dimensions of a discrete wire-wound coil represent a limitation on the overall dimensions of the sensor device package; further size reduction of the device for implantation applications always being desirable.
In view of the foregoing, there remains a need in the art for an improved implantable sensor device.